This invention relates to a method of making a folded metal bellows.
Bellows of various types are used in a wide variety of industrial machines and products. Bellows which have a closed cross section are used for a variety of applications such as protecting shock absorbers, lead screws, hydraulic rods, and various other machine parts from contaminants, and for conveying or pumping gases or fluids. Some designs are also used as flexible seals for gasses and liquids. Bellows which have an open cross section are commonly used for protecting linear ways in machine tools and similar components of industrial machinery. Prior art bellows designs can be classified into three categories: the folded bellows, which is made by forming discrete fold lines into a tube or sheet of foldable material; the corrugated bellows, which is made by permanently stretching the material significantly to form an undulating surface onto a tube or a sheet; and the layered bellows, which is made by selectively joining the edges of a stack of sheets or plates.
Folded bellows designs, to which the present invention specifically relates, are used widely in industry because they have a high extended length to compressed length ratio, and they are relatively easy to custom fabricate. Prior art fold patterns which are used for folded bellows by companies today are very traditional and have been used for centuries. A useful reference which shows the two main traditional prior art fold patterns is xe2x80x9cMaterials for Bellows Type Protective Devicesxe2x80x9d by E. A. Annenberg, E. A. Maiorova, I. M. Sokhor, in Machines and Tooling, v 33, n 11, 1962 p. 39-42. In that reference, FIG. 2 and FIG. 3 show two classic fold patterns used for forming the corners of bellows. For the sake of discussion, these fold patterns, which form the corner of a bellows, will be given names. In FIG. 3 of Annenberg et. al, each V-shaped fold pattern, which forms the corner of a bellows, will be referred to as a single inversion. In FIG. 2 of Annenberg et. al, each V-shaped fold pattern that has an additional fold connecting the wings of the V will be referred to as a classic double inversion. Single inversions and classic double inversions are widely used to form the corners of bellows in industry. Also in Annenberg et. al, FIG. 4 shows an unusual double inversion pattern which is particularly used for a hexagonal closed cross section bellows, proposed by the Russian designer Pavchinskii. Since the Pavchinskii design cannot have an arbitrary cross section, it is not used in industry.
A deficiency of single inversions and classic double inversions commonly used in industry are that when they are extended, they impose outward tilting, or inward tilting, respectively, on the bellows walls. To prevent this undesirable wall tilting when the bellows is extended, shape holding frames are used which are sewn in between adjacent folds, and floppy rubberized fabrics are used to allow the fold lines to undergo gross distortions. Using either single inversions or classic double inversions, the design paradigm has been to use shape holding frames to provide the structural rigidity, and floppy rubberized fabrics to allow gross distortions of the bellows fold lines when the bellows is extended. This prior art design paradigm means that an expensive multi-step assembly process must be used to make the bellows and expensive rubberized fabrics must be used for the folded material.
As stated earlier, all major bellows manufacturers which make protective covers use either single inversions or classic double inversions to form the corners of bellows. As a result, bellows manufacturers of today are forced to use the costly design paradigm for bellows discussed earlier. For example, in the Design Handbook provided by Milwaukee Protective Coversxe2x80x94P.E.I., Milwaukee, Wis., several way cover bellows are shown which use a classic double inversion to form the bellows corners. Page 5 of the handbook shows several PVC stiffening panels which are welded between each fold of a special flexible material. In the Product Guide supplied by Protect and Hema, L.L.C., Loves Park, Ill., on page 15 is shown the underside of a bellows that uses a classic double inversion, which has a complex assembly of stiffening panels and straps to prevent the bellows from overextending. These extra parts are needed because the folded material itself is too floppy to give the bellows support. On pages 17 and 18 are shown bellows that use single inversions to form the bellows corners and plastic stiffening panels glued underneath each bellows panel. In another example, a product flyer provided by AandA Mfg. Co., Inc. of New Berlin, Wis. entitled xe2x80x9cGortite(copyright) Linear Rail Coversxe2x80x9d shows several way covers which use classic double inversions to form the bellows corners with stiffening panels at each fold. Another manufacturer, Centryco(copyright) of Burlington N.J., in their flyer entitled xe2x80x9cCentryco Bellows Selection and Design Guidexe2x80x9d shows some closed cross section bellows which use classic double inversions to form the bellows corners with stiffening panels sewn in at each fold.
Another relevant area of prior art is passage protection devices for articulated buses or trains. In U.S. Pat. No. 5,471,934, a novel tongue and groove fold is shown to form the corner of a bellows which can provide long extension lengths. While the geometry of this fold is different than either the single inversion or the classic double inversion, it still requires a flexible fabric to be used, and stiffening frames to be attached on either side of the fold. Therefore, this fold does not change the design paradigm described for bellows and hence is costly and complex.
An object of the present invention, accordingly, is to provide an improved family of fold patterns for forming the corner of a bellows which when folded into a stiff but foldable material, for which the folds act like hinges and the bellows panels remain rigid, can be designed using a mathematical model to provide minimal tilting of bellows walls over a specified extension angle range, thus allowing, unlike all other prior art folds, a structurally stiff, long extending bellows which holds its own shape to be made from a single sheet of stiff but foldable material.
A further object is to provide an improved family of fold patterns for forming the corner of a bellows which can be designed using a mathematical model to provide exactly zero tilting of the bellows walls at one or more non-zero extension lengths specified by a designer, thus allowing, unlike all other prior art folds, a structurally stiff, long extending bellows to be formed in an extended state using fast production techniques such as vacuum forming, blow molding, or injection molding, while also allowing the bellows to be free of distortion in the compressed state.
A further object is to provide an improved family of fold patterns which when incorporated into a bellows that does have shape holding frames, greatly reduces the material stress and distortion along the fold lines when the bellows is extended, thereby allowing the bellows to be compressed and extended with less force, allowing longer extension lengths, longer fatigue life, and allowing a designer to use a greater variety of stiff but foldable materials for bellows designs, as opposed to prior art folds which restrict a designer to using only floppy materials such as fabrics and rubber sheets.
A further object is to provide an improved family of fold patterns for forming the corner of a bellows which when properly designed and optimized using a mathematical model, can provide a bellows with two or more elastically stable states; the first state being near full collapse, and the other states being near full extension, thereby providing a novel collapsible, expandable conduit or container.
A further object of this invention is to provide a method of making a folded metal bellows.
A further object of this invention is to provide such a method which is less labor intensive than the manufacture of welded bellows.
A further object of this invention is to provide a folded metal bellowsxe2x80x94a structure not before manufactured.
This invention results from the realization that a metal bellows can be more easily and thus more economically manufactured not by welding or forming techniques but instead by folding the bellows and optimizing the inversion design angles based on the maximum allowable change in extension angle using a pair of n-sided mandrels inside a metal tube to be formed into an n-sided bellows and axially moving and rotating one mandrel with respect to the other by predetermined distances and angles such that the edges of the mandrel crease the metal bellows in a predetermined way to thus form convolutions.
This invention features a method of making a folded metal bellows wherein an n-sided mandrel is positioned within a metal tube at an initial position and convolutions are formed by: 1) axially moving and turning the mandrel with respect to and within the tube from the initial position to a second position, and 2) fixing the mandrel with respect to the tube and driving the mandrel back towards the initial position while again turning the mandrel with respect to the tube. In step 1), the mandrel forms creases in the tube and in step 2) the tube is folded along the creases forming a convolution.
Further included may be the step of forming additional:convolutions by positioning the n-sided mandrel at a new initial position (e.g., the second position) within and with respect to the metal tube and repeating steps 1) and 2). When additional convolutions are formed, a second n-sided mandrel may be used and placed within the metal tube such that the convolutions are located between the second n-sided mandrel and the first n-sided mandrel. A portion of the tube beyond (e.g. above) the convolutions and the first mandrel should be fixed with respect to the second n-sided mandrel to prevent movement of convolutions during movement of the first n-sided mandrel. The number of sides (n) of the mandrel may be between four and nine.
The first n-sided mandrel is preferably turned with respect to the tube in the same direction on both the upward stroke and the downward stroke but then the turning direction reversed after forming a predetermined number convolutions to eliminate twisting of the bellows under compression.
The invention also features a folded metal bellows made by this method. In the preferred embodiment, the method of making a folded metal bellows out of a round tube in accordance with this invention comprises: choosing first and second inversion design angles X1 and X2 based on the function:
0=2xcfx80/ns-2 tanxe2x88x921(cos(1.2 xcex94xcex1max/2)tan(xcfx80/ns+"khgr"2))+2 tanxe2x88x921(cos(1.2 xcex94xcex1max/2)tan("khgr"2))
where xcex94xcex1max is the maximum allowable change in the extension angle and xcex7s is the number of sides of the bellows and       x    1    =            π              n        s              +                  x        2            .      
This invention also features a folded metal bellows made by this method. More broadly, in contrast to welded metal bellows, the method of this invention includes the steps of forming creases in a round tube and compressing the tube such that the tube material folds along the creases to form convolutions along the creases The method of making a folded metal bellows, also comprises positioning an n-sided mandrel within a metal tube at an initial position; and forming a convolution by: 1) axially moving and turning the mandrel with respect to the tube from the initial position to a second position to form creases in the tube, and 2) axially moving and turning the tube to fold the tube along the creases. The mandrel may itself be moved and rotated or a second mandrel, fixed with respect to the tube may be rotated as the first mandrel is moved axially within the tube. Preferably, the first mandrel is fixed with respect to the tube during step 2 and it is the mandrel which is axially moved and turned and brought back proximate its original position to fold the tube along the creases.